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Mileposts Along the Overseas Highway

By JOHN CUMMINGS; JOHN CUMMINGS is the author of ''Goombata,'' to be published by Little, Brown.

Published: November 20, 1988

THE highway mile markers in the Florida Keys once aided the driver as the sextant aided the explorer. Like highway mile posts in rural Alaska today, these state-posted markers were the surest reference for the traveler, and all directions in the Keys were given in mile markers. Today they are still there on the Overseas Highway, or U.S. 1, stretching about 100 miles from Key Largo to Key West. But they are obscured and largely obsolete as driver aids, just one more reminder of the Keys that are no more.

Today the traveler finds road references among the endless billboards, tacky roadhouses, fast food restaurants and supermarkets. In short, traversing much of the Overseas Highway, which strings together these subtropical islands with an ecology like no other in the United States, is at times akin to driving through the shopping-mall sprawl of North Miami.

The drive from Miami to Key West can be done in less than three hours, but the same route can also take days. For tucked away behind nearly 30 years of uncontrolled development are reminders of Old Florida, a fast-disappearing place of mangroves and panthers and unpeopled spaces. For those who would seek them out, the sanctuaries of U.S. 1 make rewarding detours.

The first stop might be Biscayne National Park, reached by exiting Florida's Turnpike at Tallahassee Road south of Homestead. Follow the signs, heading south to North Canal Drive, which is easily found because the canal runs next to it. Turn left, and the road leads to the park's ranger station.

Harry Truman went to Florida in 1947 to dedicate Everglades National Park, a move that would help focus public attention on preserving the endangered ''River of Grass.'' But there was no fanfare attending the opening in 1980 of Biscayne National Park, another vanishing ecosystem, with 40 islands and 20 miles of coral reef. Today the park staff laments the small number of visitors. On 181,500 acres, most of which are an underwater reef, the park is viewed by glass-bottom boat. The beginning of the trip is marred by the sight of the Miami skyline, visible on the northern horizon on clear days, coupled by the profile to the south of the nearby Turkey Point nuclear power plant.

Yet below the clear waters of Biscayne Bay, out of sight of man-made things, the visitor might spot rare sea turtles, which have reappeared after coming close to extinction, or the glistening fins of a queen angel fish or parrot fish. There are also green moray eels, bulbous brain corals, swirling sea fans, sponges and crabs. A Park Service naturalist explains to visitors the complete marine ecosystem. On shore, mangroves, essentially dense trees and shrubs that live in saltwater, sway in the tropical breezes. The land is dotted with ferns and several variety of palms, both stately and squat, and the sky is alive with birds.

From Biscayne National Park, it is about 30 miles south to the John Pennekamp Coral Reef State Park in Key Largo. (Pennekamp, an avid conservationist, was an editor for The Miami Herald from the 30's to the 70's.) This is what the state park rangers call ''the real Florida,'' the Florida that is disappearing. But before heading to Pennekamp, take a detour for one of the last remnants of the real old Keys. Instead of staying on U.S. 1 after leaving Biscayne National Park, turn off on State Route 905 near the town of Florida City for the toll bridge, which leads through a time warp to North Key Largo. All of the Keys resembled North Key Largo before the developers came.

Spend at least part of a day in this undeveloped area of virgin forest, mangroves and marshes. In unposted areas, where there are no trails, a walk into the woods will often reveal wild orchids. There is even a remote chance of seeing the endangered Florida Panther. There have been recent sightings in this area, state officials say. It's impossible to get lost walking, since walking in the Keys will eventually bring the hiker to a road or the water.

North Key Largo reveals to the hiker what divers see, but without having to get wet, since much of the terrain is coral rock. Robert Wilhelm, the assistant park manager at Pennekamp, says he often advises visitors to take in North Key Largo because it is itself an above-water ''reef'' of coral formations and mangroves.

The State of Florida would like to acquire some of this area, but even if the land is taken for parklands there will still be the inevitable signs of civilization, such as refreshment stands and campgrounds.

Along State Highway 905 are areas where saltwater crocodiles, among the last to be found in this country, can be seen.

From here on - near the Key Largo National Marine Sanctuary - people like to live in wet suits and underwater gear. This, America's first underwater park and Florida's most popular, is a reef on which nine spots are marked for diving and snorkeling, which are the prime reasons people go to Pennekamp. Near one reef, beneath the waters, there is a giant statue of Jesus with arms raised above His head. Pennekamp also offers a marked, self-guided canoe trail in sheltered waters, and canoe rentals are available.